N-acetyl neuraminic acid (Neu5Ac) is the most abundant derivative of sialic acid found at the terminal end of glycoproteins or glycolipids. Neu5Ac-containing conjugates on the cell surface are often the receptors for viruses and bacteria and also are involved in a variety of biological processes such as tumor metastasis, cell differentiation and cell-cell interactions.1 Influenza hemagglutinin (HA), for example, is the primary protein responsible for interacting with the sialosides on host-cell membrane to direct viral entry.2 
In naturally occurring sialosides, Neu5Ac is often linked to galactosides through the α(2→3) or α(2→6) linkage in both N-linked and O-linked glycoproteins, and also to N-acetyl-galactosamine through the α(2→6) linkage in O-linked glycoproteins. In addition, Neu5Ac can be linked to another Neu5Ac residue via the α(2→8) or α(2→9) linkage as a constituent of glycoconjugates.3 Preparation of sialosides with α-glycosidic linkage, however, still represents a major challenge, mainly due to the presence of the electron-withdrawing carboxyl group at the tertiary anomeric center and the lack of a participating group at C-3. As a result, low-yield, undesired 2,3-elimination, and the formation of unnatural β glycosidic bonds occurs. Further, the separation of α- and β-isomers is tedious and sometimes very challenging. Though enzymatic sialylation provides stereo-specific α-linked sialosides, it is limited to, with some exceptions, the synthesis of naturally occurring sialosides.4 
Over the past several years, major efforts have been directed toward the development of sialic acid donors for efficient α-sialylation5, including the use of sialyl donors with halides,6 phosphites,7 sulfides,8 xanthates,9 or phenyltrifluoroacetimidates as leaving groups,10 or with an auxiliary group at C-111 and C-3,12 or with a modified N-acetyl functional group at C-513. Another type of sialyl donors combines the best leaving group with the best positional modification for selective α-sialylation.14 To date, however, there is no general sialyl donor available for use in preparing all naturally occurring sialosides with α(2→3), α(2→6), α(2→8), or α(2→9) linkages.
Development of efficient synthesis of oligosaccharides, using armed-disarmed,15 one-pot,16 reactivity-based programmable one-pot,8b,17 solid phase,18 orthogonal,19 and pre-activation20 methods, has been reported to reduce the complexity of protecting group manipulation.17-18 However, one-pot synthesis of sialosides still presents a major problem due to the low reactivity and selectivity of sialylation reagents.14a, 21-22 A strategy based on the use of appropriate orthogonal leaving groups has been demonstrated for the stereoselective synthesis of α(2→9) pentasialic acids, in which sialyl phosphite was chosen as the donor and thiosialoside as the acceptor for repeating coupling and anomeric leaving group adjustments.14b In addition, the preparation of α(2→6)-linked sialo-trisaccharides was accomplished by the sequential assembly of three building blocks composed of a sialyl donor with S-benzoxazolyl (S-Box) leaving group, a thiogalactoside as first acceptor and a 1-O-methyl glucoside as second acceptor,23 to give the α-selective product (α/βratio of 2.2:1 to 2.7:1). Takahashi and co-workers described a related one-pot study using S-Box sialyl donor with additional 5N, 4O-oxazolidinone protection, and achieved an elegant synthesis of α(2→9) trisialic with improved α-selectivity.14e Another excellent orthogonal strategy of one-pot synthesis of α(2→3)-linked N-glycolylneuraminic acids is to use admantanyl thiosialoside as a sialyl donor.14f Therefore, there is a need to develop a new sialic acid donor with an efficient leaving group that can be used as a general donor in an orthogonal strategy for the convergent synthesis of various sialosides with different linkages.
Sialyl phosphite,7a-d has been used for selective α-sialylation, but less attention has been given to the corresponding phosphate because of its poor α-selectivity in glycosidation reactions.7a,7d 